The invention relates to a charge coupled device of the buried channel type comprising a semiconductor body which at a surface is provided with a charge transport channel in the form of a surface region of a first conductivity type which is laterally bounded in the semiconductor body by a bounding region of the opposed, second conductivity type and which is provided with a charge detector in the form of an insulated-gate surface-channel field effect transistor, a surface zone of the second conductivity type being provided locally in the charge transport channel so as to form a source zone of the field effect transistor, while potions of the bounding region of the second conductivity type on either side of the charge transport channel form a drain zone of the field effect transistor adjacent the source zone, and portions of the charge transport channel between the source and drain zones form a channel region of the field effect transistor. The invention also relates to an imaging device provided with such a charge coupled device.
A charge coupled device of this type is known inter alia from U.S. Pat. No. 4,074,302. Charge coupled devices are described therein with an n-type buried channel which is provided with a p-type insulated-gate surface-channel field effect transistor with which the size of the charge packets can be determined. The source zone may be formed by a p-type surface zone in the n-type CCD channel, while the drain zone is formed by an adjoining portion of the p-type region which forms a lateral boundary of the CCD channel. The current direction in the MOS transistor (called MOST hereinafter for short) is usually transverse to the current direction in the CCD channel. During operation, a voltage is applied to the gate electrode of the MOST in excess Of the threshold voltage of the MOST. The charge packet to be read; which consists of a packet of electrons, is transported to below the MOST channel. The concentration of the n-type doping in the CCD channel below the MOST is increased for the purpose of charge storage below the MOST. The charge packet acts as the backgate of the MOST, whose potential, i.e. number of electrons in the charge packet, modulates the conduction in the MOST channel. The value of the charge packet can be determined through measurement of the conductivity in the channel, for example in that the MOST is included in a source follower circuit. After read-out, the charge packet is further transported for subsequent processing or drained off. Since all electrons are essentially removed from the charge storage location below the MOST during this, there is no or at least substantially no reset noise such as does occur in, for example, an output with floating diffusion, where the charge is stored in an electrically floating, highly doped zone which is connected, for example, to a source follower. FIG. 4 of U.S. Pat. No. 4,074,302 shows an embodiment in which the MOST channel is limited to approximately half the width of the CCD channel, whereby the sensitivity of the detector is further enhanced. It is indeed indicated in the accompanying description that the MOST may also be made double-sided, so that the MOST will extend over practically the entire width of the CCD channel, but it is also stated that this is to the detriment of the sensitivity. The transistor in this embodiment, moreover, cannot be manufactured by self-alignment, which renders the production process more difficult, in particular as the dimensions of the device become increasingly smaller.
In the design of the charge detector, requirements as to inter alia noise behaviour, charge capacity, bandwidth, and speed are also to be met. When the detector is connected in a source follower configuration, the signal-to-noise ratio becomes generally better in proportion as the current through the transistor is greater for a charge packet of a given size, i.e. in proportion as the W/L ratio in the transistor is higher. Usually, however, an increase in the width W of the transistor is accompanied by a decrease in the sensitivity. Since a high sensitivity is usually preferred, most known configurations have a fairly indifferent quality as to their noise behaviour. Since optimization of the sensitivity often takes place to the detriment of, for example, the maximum size of the charge packet, however, it may be of major advantage to accept a somewhat smaller sensitivity if other important properties, such as the charge capacity, can be improved thereby.
In addition, known embodiments are usually designed such that the length L of the channel region is so much greater than the length L of the clock electrodes of the charge coupled device that the output stage is much slower than the remainder of the device.